C++ - std::decay<T> and std::make_tuple<D> - cannot convert argument from 'D' to 'D &&'

Question

I have this code:

template <typename T>
void someFunction(){
   typedef std::decay<T>::type D;

   D val = GetValue<D>();
   std::tuple<D> t = std::make_tuple<D>(val);

   //... tuple is stored outside this scope in global variable
}

GetValue<D>() returns value from dictionary based on typeid(D), it is working correctly and I return MyData to D - I tested it if I comment out the next problematic line, see the problem below.

If I compile this, for T = const MyData & I got this

 error C2664: 'std::tuple<MyData > std::make_tuple<D>(D &&)': cannot convert argument 1 from 'D' to 'D &&'

Why is this error produced and how to remove it? I want to store only non-ref values in my tuple, even if template T is refd'.

I can get it compile, by using std::make_tuple<D>(std::forward<D>(val)), but it calls my move ctor and I want to cal copy ctor, because val is destroyed at the end of method and so its internal data and move ctor just moved them, so they are gone.

Answer 1

You should never pass explicit template parameters to make_tuple and make_pair. Their power lies exactly in their ability to deduce types, as well as decaying them and unwrapping reference wrappers.

The following should work:

std::tuple<D> t = std::make_tuple(std::move(val)); // forward is superfluous here

or, better yet:

auto t = std::make_tuple(GetValue<D>());